Substituent Effects. XXI. Solvolysis of Benzyl Tosylates

Abstract

Abstract The rates of solvolyses of a series of substituted benzyl tosylates were determined in 80% aqueous acetone, and the substituent effect was analyzed based on the Yukawa–Tsuno LArSR Eq. The LArSR Eq. as well as Hammett–Brown treatment failed to give any simple linear plot for the effect of the full range of substituents, indicating a significant mechanistic shift. The σ+ plot gave a split pattern of parallel curvatures with significant gaps for strong π-donors, weak π-donors, and non-conjugative substituents, respectively. The widely split σ+ plot conflicts with the accepted interpretation in terms of the kc–ks mechanistic transition, since the kc–ks mechanistic change should result in a single smooth-curved plot, only if plotted against an appropriate substituent constant scale. The plots of para π-donors against an appropriate \barσ scale with r=1.3 coalesced into a single smooth curve including the meta correlation curve. A sufficiently linear LArSR plot with r=1.3 for the reactive substituents down to 4-MeS-3-CN group can be referred to the substituent effect correlation for the kc mechanism of this system. The result of a correlation analysis confirmed this conclusion, giving a best ρ value of −5.23 and an r value of 1.28 for a range of substituents more activating than 4-MeS-3-CN. The r value for the kc solvolysis of activated range of substituents was found to be identical with the r value obtained for the gas-phase stability of benzyl cation. The enhanced resonance demand (r=1.28) obtained in the benzyl solvolysis must be an intrinsic feature characterizing the nature of the transition state of kc solvolysis of benzyl tosylate, rather than a correlational artifact of the nonlinearity caused from mechanistic complexity.